1. Field of the Invention
The present invention relates to the rotor hub, rotor blades, drive shaft and controls for a helicoptor rotor by means of which the aircraft is supported aerodynamically, controlled for flight maneuvers and connected to the drive system.
This invention is particularly suited for use with helicopter rotor systems wherein conventional, rigid mechanical bearings, by which the dynamic rotor components are usually interconnected, are replaced by flexible, elastomeric bearings that provide resilient, dimensionally variable interconnections among the various components.
2. Description of the Prior Art
A helicopter rotor includes a rotor hub driveably connected on the end of a rotor shaft, which transmits power through a transmission from the engines. Rotor blades are mounted on the hub, spaced angularly from one another and extending radially outward from the axis of the hub. The rotor blades are formed with an airfoil contoured surface so that aerodynamic lift forces are produced as the blades rotate through the atmospheric air. The lift force and a vertically directed component of the centrifugal forces of the rotor blades operate to support the aircraft and horizontal or forward directed components of the aerodynamic lift and centrifugal forces operate to accelerate and manuever the aircraft. The magnitude of the lift force varies as a function of the velocity of the rotor blades relative to the ambient air and the angle of attack or pitch angle of the blades relative to the airstream.
For a particular blade pitch angle, the lift developed by a rotor blade is greater as it advances due to rotation toward the airstream created by the forward speed of the aircraft than the lift developed as the blade retreats due to rotation from the airstream. The velocity of the airstream relative to the advancing blade is greater than its velocity relative to the retreating blade because the speed due to rotation adds to this speed due to forward flight, but as to the retreating blade, these speeds are opposed. To accommodate the speed difference and to maintain the lift force more nearly equal on the blades around the entire rotational path, the rotor controls increase the pitch angle on the portion of the rotor where the speeds are subtractive and decrease the pitch angle where the speeds are additive.
In conventional rotor systems the pitch angle change is made by pitch links fixed at one end to a gimballed pitch ring, which is tilted with respect to the rotor hub, and fixed at the opposite end to a pitch arm, which is connected to the root of the rotor blade eccentric of the blade pitch axis. The blade pivots about the pitch axis in accordance with the tilt of the pitch ring because the blade is pivotably connected to the hub by a torsionally flexible tension-torsion pack through which the centrifugal force is carried rigidly to the hub.
The rotor blades also flap about horizontal axes located at the blade root so that bending moments about the flap axis are controlled to an acceptable magnitude. The flapwise pivoting conventionally occurs by mounting the blade on bearings supported on a horizontal pin carried on the rotor hub. This pinned connection, located at a predetermined distance from the rotor center, assures that the flapwise moment at the connection is zero and establishes the magnitude of the rotor moment at the rotor center as the product of the perpendicular force on the rotor arm times the distance from the rotor center to the flapwise pinned connection.
The rotor blades also pivot about a vertical axis located a predetermined radial distance from the rotor center. In conventional rotor systems a damper is connected across this to tune the rotor system against latent instabilities that can arise due to starting the rotor while the helicopter is elastically supported on ground wheel oleos, a condition called ground resonance. The presence of the vertical or lead-lag axis assures that the moment at the connection and at the rotor center is controlled to an acceptable magnitude.
Recently, more advanced rotor systems have substituted structural flexibility for the mechanical pinned rotor blade connections at its root end attachments to the rotor hub about the flapwise axis, the lead-lag axis and the pitch axis, while maintaining the rigid force continuity essential to proper operation. For example, instead of the horizontal bearing connection the rotor blade has been formed with a member, located near the rotor, having relatively low flapwise bending stiffness and high axial or spanwise tension stiffness. This member, called a flexure carries all of the blade loads to the rotor yet it permits the blade to pitch, flap and lead-lag without providing bearing connections to the rotor hub to control the magnitude of the rotor moment.
In rotors of this kind, the flexure is made from fibers such as fiber glass, Kevlar, graphite, Boron, etc. supported and connected by polymeric resin matrices of materials such as epoxy, phenolic, polyesters, etc. However, although the flexure is flexible with respect to flapwise bending, edgewise bending and torsion, it must have adequate strength to transmit flapwise bending moments, edgewise or lead-lag bending moments, blade pitch or torsion moments and centrifugal force from the blade to the rotor hub.
Furthermore, the flexure cannot duplicate entirely the effect of the conventional pinned connections of the rotor blade to the rotor hub because of the structural continuity that must be maintained. Therefore, the flapwise bending moment in particular is larger than if a flapwise pinned connection were used. Accordingly, the rotor hub moments are generally higher for the advanced rotor hub systems that employ structural rotor blade flexibility as a means to simulate the effect of conventional pinned rotor blade support on the rotor hub.
When the rotor blade root end bending moments are large the rotor moment is large and requires additional control force to adjust the attitude of the helicopter rotor with respect to the attitude required for a maneuver or flight speed change.
A rotor system of the flexural type is described in the paper entitled "Flexible Matrix Composite Applied to Bearingless Rotor Systems", by A. J. Hannibal et al, presented at the American Helicopter Society Composite Structures Specialists Meeting in Philadelphia, Pa., March 1983. U.S. Pat. Nos. 3,669,566 and 4,332,525 describe helicopter rotor systems that employ composite materials.